1. Field of the Invention
The present invention relates to a disc brake for a vehicle, such as an automobile or a motorcycle. More particularly, to piston seals and seal grooves for the disc brake.
2. Description of the Background Art
For this type of brake, a brake piston slides in and out of a caliper at the driver's command and places pressure on the disc brake to stop or slow the vehicle. For example, a disc brake is disclosed in Japanese published unexamined patent application No. Hei 9-72361.
As shown in FIG. 10(a), a brake piston 02 is inserted into a cylinder 01 of a caliper for slideably mounting the brake piston. Located on the left side in FIG. 10(a), a pad and others are pressed upon a brake disc, not shown, by applying hydraulic pressure to the other side (the right side in FIG. 10(a)) of the brake piston 02. As a result, the rotation of the brake disc is stopped. A piston seal 03 made of rubber and having a rectangular or trapezoidal section is set on a seal groove 04 that works on the inner surface of the cylinder 01 of the caliper. A large clearance (space) 04a is provided in the seal groove 04 in a direction in which the brake piston 02 is slid (particularly on the hydraulic side of the brake piston 02).
In the conventional type disc brake, when the temperature of the cylinder 01 rises, the piston seal 03 is also heated, and as shown by black arrows in FIG. 10(b), the piston seal is expanded towards the clearance 04a on the side of the application of hydraulic pressure in the seal groove 04. Consequently, the brake piston 02 is also pushed back on the side of hydraulic pressure as shown by a white arrow in FIG. 10(b) and the amount of play of the brake is increased. More specifically, due to the play in the brake, braking effectiveness will vary.
In the conventional type disc brake, air is apt to accumulate in the large clearance 04a in the seal groove 04 and the braking effectiveness (the touch feeling of the brake) is deteriorated. In addition, as shown by an arrow in FIG. 11, when the piston seal 03 which has a rectangular section is inserted into the seal groove 04, there is also a problem that a corner 05 may be caught and the piston seal 03 is often inserted with a twist.
The temperature of the cylinder 01 may sometimes exceed 100 degrees Celsius. It is known that the condition of the grip of a brake lever varies when the temperature rises, as described above. This variation in the condition of the grip is considered to be mainly caused by the thermic deflection of the cylinder 01 and the brake piston 02. However, as a result of examination and research by the present inventors, the main cause is that the brake piston 02 is pushed back by the thermal expansion of the piston seal 03. When the temperature of the cylinder 01 rises up to approximately 100 degrees Celsius, the brake piston 02 is pushed back up to approximately a few tens μm.